High power visible laser with a laser-fabricated non-linear waveguide

ABSTRACT

Novel methods and systems for waveguide fabrication and design are disclosed. Designs are described for fabricating ridge, buried and hybrid waveguides by a femtosecond pulsed laser. A laser system may combine a diode bar, a wavelength combiner and a waveguide. The waveguide may convert the electromagnetic radiation of an infrared laser into that the visible-wavelength range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/019,601, filed on Jul. 1, 2014 and U.S. Provisional PatentApplication No. 62/035,674, filed on Aug. 11, 2014, each of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to optical waveguides. More particularly,it relates to a high power visible laser with a laser-fabricatednonlinear waveguide.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIG. 1 illustrates an exemplary process for ridge, buried and hybridwaveguide designs.

FIG. 2 illustrates an exemplary ridge waveguide.

FIG. 3 depicts an exemplary buried waveguide.

FIG. 4 depicts an exemplary hybrid waveguide.

SUMMARY

In a first aspect of the disclosure, a wavelength-converting nonlinearridge waveguide is described, the wavelength-converting nonlinear ridgewaveguide comprising: a nonlinear crystal substrate; a first dielectriclayer on a top surface of the nonlinear crystal substrate; a waveguidelayer on a top surface of the first dielectric layer, the waveguidelayer comprising at least one laser-fabricated ridge region, the atleast one laser-fabricated ridge region being configured to allowformation of an optical mode, the optical mode substantially delimitedby the at least one laser-fabricated ridge region and the firstdielectric layer; and a second dielectric layer on a top surface of thewaveguide layer.

In a second aspect of the disclosure, a wavelength-converting nonlinearburied waveguide is described, the wavelength-converting nonlinearburied waveguide comprising: a nonlinear crystal substrate; and aplurality of laser-fabricated damage tracks in the nonlinear crystalsubstrate, wherein the plurality of laser-fabricated damage tracks havea crystalline structure different from an undamaged crystallinestructure of the nonlinear crystal substrate, the plurality oflaser-fabricated damage tracks being arranged in a desired patternconfigured to allow formation of an optical mode within an area of thenonlinear crystal substrate substantially delimited by the desiredpattern.

In a third aspect of the disclosure, a wavelength-converting nonlinearhybrid waveguide described, the wavelength-converting nonlinear hybridwaveguide comprising: a nonlinear crystal substrate; a layer oflaser-fabricated damage tracks in the nonlinear crystal substrate,wherein the laser-fabricated damage tracks have a crystalline structuredifferent from an undamaged crystalline structure of the nonlinearcrystal substrate; at least one laser-fabricated ridge region on a topsurface of the nonlinear crystal substrate, the at least onelaser-fabricated ridge region being configured to allow formation of anoptical mode, the optical mode substantially delimited by said at leastone laser-fabricated ridge region and the layer of damage tracks; and adielectric layer on a top surface of the nonlinear crystal substrate.

DETAILED DESCRIPTION

Low-cost, high-power visible laser sources are an important componentfor the next generation cinema projectors. The present disclosuredescribes methods for the fabrication of such a laser. The laser isbased on a laser diode bar, a wavelength combiner and awavelength-converting nonlinear waveguide fabricated using femtosecond(fs) laser writing techniques. Laser diode bars, wavelength combinersand wavelength-converting nonlinear waveguide fabricated using plasmaetching as well as their combination are described for example in U.S.Pat. No. 7,423,802 B2, U.S. Pat. No. 7,265,896 B2, U.S. Pat. No.7,265,897 B2, U.S. Pat. No. 7,116,468 B2, US 20070297732 A1, US20080025350 A1, the disclosure of all of which is incorporated herein byreference in their entirety. Various designs of wavelength-convertingnonlinear waveguide fabricated using fs laser writing techniques aredescribed for example in Xu et al., Phys. Status Solidi RRL 7(11) (2013)1014-1017, Burghoff et al., Appl. Phys. A 89, 127-132, (2007), Dong etal., IEEE Journal Of Selected Topics In Quantum Electronics, Vol. 15,No. 1, January/February 2009, Fu et al., Optics Express, Vol. 17, No.14, 6 Jul. 2009, p. 11782, the disclosure of all of which isincorporated herein by reference in their entirety. However, suchdesigns are of limited utility for making large-mode-area, single-mode(or nearly-single-mode), low loss waveguides that can be produced involume.

A laser source in the visible wavelength range, as described in thepresent disclosure, comprises of an infrared laser light source (orsources), comprising in several embodiments a laser diode bar. Theoutput of the laser diode can be coupled to input ports of a wavelengthcombiner (e.g. an arrayed waveguide grating) that directs laserradiation to a single output port, for example, a single-mode waveguide.A wavelength-converting (frequency-doubling) nonlinear waveguide canthen convert the infrared radiation into the visible spectral range. Thewavelength combiner and/or the nonlinear waveguide can provide opticalfeedback to the laser diode, stabilizing operation of each emitter atits own wavelength.

The nonlinear waveguide can be fabricated using fs laser writingtechniques. This allows for a better performance and lower productioncosts compared to plasma-etching of waveguides and compared to othersystems. The nonlinear waveguide can be fabricated on a stoichiometric(or nearly stoichiometric) lithium tantalite substrate (SLT). SLT has ahigh nonlinear optical coefficient and allows for efficient wavelengthconversion and high power handling capabilities (>10 W). For highconversion efficiency, the substrate can be z-cut (x-cut and y-cut couldalso be used) and may have a quasi-phase-matching grating (with anappropriately-chosen period) that could be fabricated using commonmethods. The waveguide direction can be along the y-axis of thesubstrate. However, waveguides oriented along the x-axis could also beused. Moreover, curved or other non-straight waveguides or waveguidecircuits can be used as well. The waveguide can support single-mode (ornearly-single-mode) operation at the diode laser wavelength and/orfrequency-doubled wavelength. The supported mode diameter is 15-50 um,preferably 30 um (um being micrometers). The waveguide propagation losscan be <3 dB/cm, for example <1 dB/cm, or <0.3 dB/cm.

Three types of embodiments for a waveguide design type are described inthe present disclosure, a ridge waveguide, a buried waveguide, and ahybrid waveguide.

The cross-section of an exemplary ridge waveguide is shown schematicallyin FIG. 2. Congruent lithium tantalite (CLT) substrate (205) is coatedwith a thin layer of silica (SiO2), (210). SLT (215) is then bonded ontop and subsequently lapped uniformly across the surface to a thicknessof 15-50 um. Femtosecond laser processing is then used to remove 5-20 umof material from the top surface to leave a long ridge (220) that is5-20 um high and 20-50 um wide. The ablated zone (225) extends 50-100 umor more away from the ridge on both sides. The whole structure can thenbe overcoated with SiO2. Optionally, an additional annealing and/orplasma etching step can be performed to improve propagation losses.

In some embodiments, the substrate can be z-cut stoichiometricperiodically-poled lithium tanatalate (SLT). in some embodiments, theroughness can be <100 nm root mean square (rms), that is, the sidewalland the floor extend to 50-100 um or more outside of the ridge (220).

The ridge structure as described, for example, in FIG. 2 can support asingle-mode (or nearly single-mode) operation if the ridge height andwidth are chosen appropriately. The mode is shown schematically in FIG.2 as a dotted line (235). For simplicity, an elliptical line (235) isdrawn, whereas a real mode can exhibit a more complicated profile.

Propagation loss in the ridge waveguide is caused by scattering due toroughness and absorption. A roughness of <100 nm rms can be achieved onthe laser processed surfaces and is advantageous for the operation ofthe device. Chemical reduction of the processed surfaces may causeexcessive roughness, in contrast to the physical processing caused by alaser application. Amorphization of the SLT substrate can also beadvantageously minimized or avoided during laser processing. For lowpropagation loss, fs laser processing can be carefully optimized amongthe following parameters: scan speed, overlap, number of passes, pulselength, repetition rate, wavelength, polarization, focusing conditions,and others. In some embodiments, only the ridge of the waveguide isfabricated by a pulsed laser.

Additionally, during fs laser processing, surface environment controlscan be used to remove debris, improve roughness, control temperature andchemical composition. Such controls can be implemented via one or morejets of oxygen, ozone, nitrogen, air or other gasses.

The person skilled in the art will understand that the example of FIG. 2may be modified to obtain different variants of ridge structures.

The cross-section of an exemplary buried waveguide is shownschematically in FIG. 3. The waveguide structure is written directly inthe substrate material. Fabrication process for the buried waveguide canbe greatly simplified compared to the ridge waveguide of FIG. 2, asseveral critical and involved processing steps can be eliminated.

A permanent material damage can occur when a laser pulse (from a fslaser) with uJ-level energy is focused under the surface of thesubstrate (305) to a spot with a diameter of few microns (uJ being microJoule). The damage spot can have an oblong shape (see for example 310),about 2 by 10 um in size (or in the range of 2-10 um to 3-20 um). The fslaser beam (or the substrate (305)) can be translated perpendicular tothe beam propagation direction, resulting in a continuous or discretedamage track, see for example (315, 320, 325).

In other words, a plurality of damage tracks (310) can be traced in apattern which constitutes a waveguide.

The refractive index of the material is decreased within the track,compared to the index of the substrate (305). In the vicinity of thetrack the index is increased due to stress, as the mechanical stressdeforming the crystal lattice will change its properties, such as therefractive index.

A waveguide structure can be formed by arranging many closely-spaceddamage tracks in a circular pattern, with spacing between the trackscomparable to the individual track size.

The optical mode (330) is situated in the undamaged substrate materialinside the damage ring, that is the circular pattern formed byindividual damage tracks. Such waveguides are termed as depressedcladding or Type III waveguides. While such waveguide designs supportsingle-mode operation for small damage ring diameters, the waveguidesbecome multi-moded for larger diameters. The person skilled in the artwill understand that, for high power applications, single-mode operationand large mode diameters (>20 um, preferably >30 um) are required.

Optical fibers, large-pitch leakage-channel fiber designs can be used toextend single-mode operation to large mode diameters. In such designs,reduced-index rods (few 10s of microns in diameter) are placed aroundthe fiber core, effectively suppressing propagation of higher-ordermodes. Stress-induced guiding in optical fibers can be used to extendsingle-mode operation to large mode diameters.

In the embodiments exemplified by FIG. 3, damage tracks can be arrangedin a circular or non-circular pattern, and can be regularly ornon-regularly spaced. One or more rings of damage tracks can be used. InFIG. 3, the center of the waveguide core (335) can be 30-100 um belowthe substrate surface and the diameter of the damage ring (inside thering of tracks (310)) can be 30-100 um. The optical mode (330) issituated in the undamaged substrate material inside the damage ring(inside the ring of tracks (310)). The optical mode is shownschematically in FIG. 3 as a dotted line (330). For simplicity, acircular line is drawn, whereas a real mode can exhibit a morecomplicated profile. In some embodiments, the substrate may betransparent to the machining laser, in order to avoid absorbance of thelaser from the substrate.

Damage tracks (310) can be arranged in groups of 1-10 or more tracks.Within each group the tracks can be placed in proximity of each other,with separation comparable to the track size. This leads to an effectiverefractive index reduction over the area occupied by the group. Thegroups can be separated by a larger distance (comparable to the size ofthe group) to suppress higher-order mode propagation, similarly tolarge-pitch leakage channel fiber designs. In some embodiments, 4 to 36or more groups can be used.

Alternatively, a stress-induced increase of the refractive index in thevicinity of damage tracks can be effectively used to form a waveguide.This can be achieved by placing damage tracks around the undamaged core,thereby causing an increase in the refractive index of the undamagedcore, due to the stress in the crystal lattice. Placement of the damagetracks, their strength and the resulting stress distribution cart beoptimized for single-mode, large mode diameter waveguide operation. Thestress distribution around a damage track can generally be anisotropic.It is advantageous to take the anisotropicity into account in thewaveguide design. In such designs, the optical mode field can beeffectively pushed away from the damage tracks. This can result inreduced propagation loss as well as in improved nonlinear conversionefficiency.

Yet another alternative is to utilize both leakage-channel andstress-induced guidance to achieve low loss, single-mode, large modediameter waveguide operation.

The overall process of fabricating buried waveguides may comprisecreating damage tracks under the substrate surface with a fs laser. Anadditional annealing step can be used to improve the waveguideperformance. Such fabrication process for the buried waveguide can begreatly simplified compared to the ridge waveguide fabrication. Severalcritical processing steps as well as some capital equipment areeliminated. Also, since there is no surface ablation during buriedwaveguide fabrication, there is no need to control a complicated surfacechemistry.

The fs laser processing parameters can be selected to produce a strongreproducible damage track while avoiding material fracture at and aroundthe track. To obtain the best waveguide performance, the parameters canbe carefully optimized among the following: scan speed, overlap, numberof passes, pulse length, repetition rate, wavelength, polarization,focusing conditions, and others.

In the embodiments above, the fabrication of continuous damage trackshas been described, as can be done by overlapping damage spots producedby individual fs pulses. Alternatively, effective damage tracks can beproduced by placing individual non-overlapping damage spots in a regularpattern along the waveguide direction, as shown in FIG. 3. Exemplarypatterns (315, 320, 325) as depicted in FIG. 3 may all be used, as wellas other variations as understood by the person skilled in the art. Aregular pattern of damage spots has well-defined spatial frequencieswith low losses between the peaks. Appropriate choice of spacing betweenthe damage spots can lead to a lower waveguide propagation loss. Sucharrangements can also be used in the fabrication of hybrid waveguidesdescribed below.

The substrate in FIG. 3 can be z-cut stoichiometric periodically-poledlithium tanatalate.

The cross-section of an exemplary hybrid waveguide is shownschematically in FIG. 4. In a first step, an array of damage tracks canbe produced using a fs laser at, for example, a depth of 15-50 um belowthe surface of a SLT substrate (405). The tracks collectively amount toa plane of damaged material (410) that is 100-300 um or more wide. Thetracks can be regularly or non-regularly spaced. In some embodiments,the tracks are placed in proximity of each other, with a separationcomparable to the track size. Alternatively, the tracks can be arrangedin groups of 1-10 or more tracks with a larger separation between theadjacent groups (comparable to the size of the group). This can beadvantageous for further suppression of the propagation of higher-ordermodes.

In a second step, fs laser processing can be used to remove 5-20 um ofmaterial from the top surface to leave a long ridge (415) that is 5-20um, high and 20-50 um wide. The ablated zone (420) can extend 50-100 umor more away from the ridge on both sides. The whole structure can thenbe overcoated with SiO2 (425). Optionally, an additional annealingand/or plasma etching step can be performed to improve waveguideperformance. In sonic embodiments, a roughness of <100 nm rms can beachieved on the laser processed surfaces in the second step. In someembodiments, chemical reduction of the processed surfaces can beminimized or avoided. Amorphization of the SLT substrate can also beminimized or avoided during laser processing. Additionally, during thesecond step, surface environment controls can be used to remove debris,improve roughness, control temperature and chemical composition, Suchcontrols can be implemented via one or more jets of oxygen, ozone,nitrogen, air or other gasses.

The waveguide mode is situated in the undamaged substrate material, asshown schematically in FIG. 4 as a dotted line (430). For simplicity, anelliptical line is drawn in FIG. 4, whereas a real mode exhibits a morecomplicated profile.

Alternatively, a hybrid waveguide can be fabricated by first producing aplane of damaged material using the same process as described inparagraph [0035]. In a second step, plasma etching, instead of laserablation, can be used to fabricate the ridge structure.

To obtain the best waveguide performance, the fs laser processingparameters can be carefully optimized separately for both steps amongthe following: scan speed, overlap, number of passes, pulse length,repetition rate, wavelength, polarization, focusing conditions, andothers.

Fabrication process for the buried waveguide can be simplified comparedto the ridge waveguide, as bonding and lapping steps are eliminated.

As the person skilled in the art will understand, the fs laserprocessing steps for both ablation and damage track formation describedabove could alternatively use few-picosecond pulses and even longerpulses, up to few nanoseconds, and still achieve required waveguideperformance. For example, the use of sub-nanosecond pulses in the UVspectral range (ex. at about 355 nm) can allow the achievement of lowdamage/ablation processing thresholds.

Ridge, buried and hybrid waveguide designs as described in the presentdisclosure can be constructed on a stoichiometric (or nearlystoichiometric) lithium tantalite substrate. It is understood that suchwaveguides can be constructed on other nonlinear substrates (forexample, congruent lithium tantalite, lithium niobate, KTP, BBO, LBO,BiBO, etc.). It is further understood that such waveguides can becontracted on other crystalline, non-crystalline or ceramic; doped orundoped substrates (Nd:YAG, glass, etc).

FIG. 1 describes an exemplary process for ridge, buried and hybridwaveguide designs as described in the present disclosure. For example,lapping, bonding and fs laser machining may be used to fabricate thewaveguides.

The surface of the devices, prior or during fabrication, may be cleanedwith various methods, such as cleaning or oxidation jets. For example,an oxidation jet may be a jet of reactive gases which clean the surfaceof the substrate by chemical reactions with surface contaminants and/ordebris.

The waveguides fabricated according to the methods and designs of thepresent disclosure may be used to fabricate a laser system. The lasersystem may comprise a laser based on an infra-red diode bar, and alaser-fabricated waveguide. The waveguide can be used for the frequencyconversion of the infrared laser into the visible wavelength. Suchlaser-fabricated waveguides can offer better performance and lowerproduction costs compared to plasma-etched waveguides. The laser systemmay also include a wavelength combiner, for example a waveguide grating,such as an arrayed waveguide grating.

As understood by the person skilled in the art, a diode bar is an arrayof diodes.

As understood by the person skilled in the art, arrayed waveguidegratings can be used as optical multiplexers in wavelength divisionmultiplexed systems. Arrayed waveguide gratings multiplex a number ofwavelengths into a single optical waveguide, thereby increasing thetransmission capacity of an optical channel.

The arrayed waveguide gratings are based on a fundamental principle ofoptics that light waves of different wavelengths interfere linearly witheach other. For example, if each diode in a diode bar emits light of aslightly different wavelength, then the light from a large number ofthese channels can be carried by a single optical waveguide (channel)with negligible crosstalk between the channels.

In some embodiments, the geometry of the waveguides of the presentdisclosure is configured to support single-mode (or nearly-single-mode)operation. The processing parameters (including, but not limited to fslaser processing parameters) are optimized for waveguide performance.The fs laser processing parameters are optimized among the followingparameters: scan speed, overlap, number of passes, pulse length,repetition rate, wavelength, polarization, focusing conditions, andothers.

In some embodiments, depending on the wavelength of the machining laser,the substrate may be transparent or absorbing for some or all of thefabrication steps. It may be advantageous to use a shorter wavelength,for example, to obtain a smoother surface for ridges in a hybridwaveguide (compared to other process steps, such as damage trackformation).

In some embodiments, the substrate does not need to be transparent. Forexample, for a ridge waveguide, the substrate does not need to betransparent for the machining laser. In other embodiments, the substratemay need to be transparent. For example, for a ridge waveguide, it maybe possible to create the ridges on one side (for example, the frontside) with a laser illuminated through the substrate from the other side(e.g., the back side). In another example, for a hybrid waveguide, thesame laser can be used to create the ridges and damage tracks from thebackside of the substrate. This method can allow the substrate to hemounted once to the fixture, thus improving design tolerances.

In several embodiments, a smooth ridge surface may be desirable.However, the ridge surface may be rough due to process of fabrication.In order to improve smoothness (arid desired planarity characteristics),it is possible to pre-treat or post-treat the surface of the material tobe laser-machined. For example, a pre-treatment may comprise ametallization step, or painting step with a compound. The pre-treatmentcan provide local absorption of the machining laser (which focuses theenergy to the top surface).

Post-treatment may comprise chemical etching for smoother (or moreuniform) surface. Etching may comprise, for example, the use of HF(hydrofluoric acid), nitric acid HNO3), ammonium fluoride (NH4F) and/orpotassium hydroxide (KOH), ethylene diamine, tetramethylammoniumhydroxide (TMAH). Plasma etching (e.g. chlorine or fluorine), may alsobe used for smoothing a surface. For example, fluorocarbon (CF4) may beused for smoothing ridges.

Several references are cited in the present disclosure, the disclosureof all said references is incorporated herein by reference in theirentirety.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

The examples set forth above are provided to those of ordinary skill inthe art as a complete disclosure and description of how to make and usethe embodiments of the disclosure, and are not intended to limit thescope of what the inventor/inventors regard as their disclosure.

Modifications of the above-described modes for carrying out the methodsand systems herein disclosed that are obvious to persons of skill in theart are intended to be within the scope of the following claims. Allpatents and publications mentioned in the specification are indicativeof the levels of skill of those skilled in the art to which thedisclosure pertains. All references cited in this disclosure areincorporated by reference to the same extent as if each reference hadbeen incorporated by reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

1. A wavelength-converting nonlinear ridge waveguide comprising: anonlinear crystal substrate; a first dielectric layer on a top surfaceof the nonlinear crystal substrate; a waveguide layer on a top surfaceof the first dielectric layer, the waveguide layer comprising at leastone laser-fabricated ridge region, the at least one laser-fabricatedridge region being configured to allow formation of an optical mode, theoptical mode substantially delimited by the at least onelaser-fabricated ridge region and the first dielectric layer; and asecond dielectric layer on a top surface of the waveguide layer.
 2. Thewavelength-converting nonlinear ridge waveguide of claim 1, wherein thenonlinear crystal substrate is congruent lithium tantalite.
 3. Thewavelength-converting nonlinear ridge waveguide of claim 1, wherein thefirst and second dielectric layers are silicon oxide.
 4. Thewavelength-converting nonlinear ridge waveguide of claim 1, wherein thewaveguide layer is lithium tantalite.
 5. The wavelength-convertingnonlinear ridge waveguide of claim 1, wherein the at least one ridgeregion has a height between 5 and 20 micrometers, a width between 20 and50 micrometers, and a distance between the top surface of the ridgeregion and the top surface of the first insulating layer is between 15and 50 micrometers. 6-24. (canceled)